In the art related to chemical treatment, wherein fluids are utilized for various purposes, including but not limited to etching and redox (oxidation-reduction) treatments, and generally in systems where ion concentration needs to be controlled, various techniques have been developed. Some of these techniques have addressed themselves to pH control of etching solutions and the like, as well as to specific gravity control. Some types of etching equipment to which such controls are adaptable are for example those disclosed in U.S. Pat. No. 4,015,706 to Daniel L. Goffredo and Conrad Dale Shakley.
In various types of chemical treatment, particularly in the etching or chemical machining of articles, for example, printed circuit boards, the articles are delivered along a conveyor through an etching chamber in which they are sprayed with an etchant. In instances in which the articles being treated contain copper, and the etching solution is being used to remove some of the copper from the articles, the etching solution may for example be ammonium hydroxide and ammonium chloride with a chelating or complexing agent, (generally with cupric ions at the beginning of the reaction to initiate oxidation). As etching continues, there is a build-up of copper ions in the solution that tend to use the etchant and etchant may be lost to atmosphere by evaporation. Consequently, it becomes necessary to periodically (or continuously) add a replenisher solution to the etchant. A typical replenisher solution would be additional ammonium chloride and ammonium hydroxide with a chelating or complexing agent. During the etching operation, the vaporizing of ammonia tends to reduce the pH of the etching solution. Consequently, it becomes necessary to make a correction, that will raise the pH of the solution to the desired level. This can be done by the addition of ammonia to the etchant. In unsophisticated operations, when the pH dropped to an undesired level, the etchant would merely be discarded and a new batch utilized. However, economic factors dictate that this is an undesirable procedure. Consequently, the practice developed whereby the lost ammonia was replaced by adding aqua ammonia (ammonium hydroxide) to the etchant which diluted the etchant, thereby generally interfering with specific gravity control. Some attempts were made to improve upon the situation by adding anhydrous ammonia.
The desirability of pH control was noted, so that anhydrous ammonia could be added automatically. However, it was observed that with the loss of ammonia and the lowering of pH, copper precipitates in the form of complex salts would tend to clog nozzles, gears, and pumps, such that there would cease to be any effective operation of the pH controller. Such precipitates furthermore, interfered with the operation of the etching operation in general.
Where the pH controllers involved the use of electrodes, the precipitation of salts described above would tend to foul the electrodes. This situation would especially occur during overnight or weekend shutdowns of the etching apparatus when the ammonia gas escaped from the etchant in larger quantities, causing the pH to drop, which brought on the precipitation. Upon startup, the electrodes would be fouled and the control system could not function properly. Proper operation of such a system was entirely dependent upon operator attention to properly cleaning electrodes.